This is what I was thinking of when I wrote what I did - The Coinage Act of 1792: Congress defined the dollar as being 371.25 grains of silver. It then regulated the value of gold coins at 24.75 grains. This means you could exchange 15 grains of silver for every grain of gold.
Nevertheless, the Constitution didn't give Congress the power to delegate its authority to define the dollar and regulate its value to a third party bank otherwise known as the Federal Reserve.
Well, you're right about that. In fact, if you've ever read any of the Founding Father's writings on this subject, the concept of a central bank was hated and feared as being inherently tyrannical in nature.